1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device for improving the leak current of a photo diode formed on a silicon Very-LSI, a solid-state image pickup device using the same, and method of manufacturing the same.
2. Description of the Related Prior Art
In order to convert light into an electric signal, a photo diode is generally used. A photo diode formed on a silicon substrate allows an signal converted into an electric signal to propagate and leak through the silicon substrate. This leak signal is called a dark current. As the dark current increases, signal portions of abnormal level not corresponding to the quantity of light to be used in small quantity of light are generated, and it cannot be used as a normal device. As causes of such dark current, the interface state density of the silicon substrate surface and process induced defect are considered. Hence, to suppress the dark current, it is necessary to prevent the effects of interface state density on the silicon substrate surface and crystal defect. Accordingly, an impurity layer of opposite conductive type for photo diode is formed on the surface of the photo diode. Therefore, when a voltage is applied to the device, the depletion region extended from the photo diode is prevented from growing up to the surface side.
Concerning this conventional method, its structure is described below by reference to FIGS. 14 and 15. On a P-type silicon substrate 1, an oxide film 2 containing silicon oxide film or silicon nitride is formed. A mask 3 is formed on the oxide film 2. After opening the specified position of the mask 3, a phosphorus ion beam 4 is selectively implanted. By subsequent heat treatment, phosphorus ions are diffused to a depth depending on the characteristics. By this phosphorus ion implantation, an N type diffusion layer of a relatively low concentration is formed as a photo diode 5.
Furthermore, as shown in FIG. 15, a boron ion beam 7 is implanted in the silicon substrate surface to form a P type diffusion layer 6.
An oxide film, electrodes, a metal wiring are further formed thereon to form a device in a desired structure. As an example of this element, FIG. 16 shows a sectional structure of a photo diode and reading part in a solid-state image pickup device of CCD (charge coupled device) type. A P type well 11 is formed on a silicon substrate 10, and a photo diode 12 and a transfer channel 13 are formed thereon. A gate dielectric film 14 is formed on the silicon substrate 10. A transfer gate electrode 15 is formed on the transfer channel 13 through the gate dielectric film 14. A dielectric film 16 is formed on the transfer gate electrode 15. A metal wiring 17 is formed thereon. Further thereon, a flattened film 18 is formed. A color filter 19 is formed thereon, and a micro lens 20 of resin material is formed thereon.
In such structure, the light condensed by the micro lens 20 accumulates electrons in the photo diode 12, and applies a voltage for reading to the transfer gate electrode 13. As a result, the electrons are read out from the photo diode 12 to the transfer channel 13. An image can be composed by disposing a plurality of the photo diodes 12 vertically and laterally. At this time, P diffusion layers 21 are formed on the surface of the photo diodes 12. when the photo diode 12 of N type diffusion layer is formed in contact with the interface between the silicon substrate 10 and gate dielectric film 14, a leak current is generated by the effect of the interface level existing on the interface. As a result, the characteristics of the solid-state image pickup device deteriorate. however, if attempted to introduce the impurity concentration of the P type diffusion layer 21 by ordinary ion implantation method, an injection defect may occur on the surface of the silicon substrate 10 by ion implantation, which may cause a leak current. Consequently, in spite of the efforts to prevent leak current, sufficient improvement effect is not obtained. To prevent such occurrence of injection defect by ion implantation, the hydrogen annealing method is effective. The hydrogen atoms introduced into the silicon substrate 10 by hydrogen annealing slip out of the interface between the silicon substrate 10 and gate dielectric film 14 when heat and light are added to the silicon substrate 10. Hence, the stability of the device is poor, and a problem occures in the reliability if storing the device for a long time after manufacture.
As other method of reducing the interface state density, a method of introducing fluorine atoms is proposed. For example, it is reported in the Japanese Journal of Appl. Physics, Vol. 28, No. 6, 1989, p. 1041. It is also reported, by G. S. Virdi et al., in the Solid-State Electronics, Vol. 34, No. 8, p. 889. Accordingly to them, it is reported that the interface state density can be reduced by injecting fluorine atoms. In this case, for fluorine injection, by using fluorine molecule ions of boron, such as BF.sub.2 and BF ions as the ion seeds, fluorine atoms are introduced into the silicon substrate 10 together with boron atoms. For example, using BF.sub.2, ion implantation is effected at the acceleration energy of 155 keV and dose of 1.times.10.sup.14 cm.sup.-2. The depth distribution of impurity concentration of boron and fluorine at this time is equivalent to the acceleration energy of 35 keV by ion implantation of boron alone, and fluorine corresponds to the acceleration energy of 60 keV.
In the constitution of the prior art, however, a defect layer by fluorine is formed nearly to the same depth as the boron profile. Afterwards, if the injected region by boron ion is widened by heat treatment, the defect layer introduced by fluorine ion implantation is seated at a deeper position. Accordingly, a voltage is applied to the N type diffusion layer of the photo diode 12, the depletion layer of the N type diffusion layer is elongated, growing up to the defect layer by fluorine. When the depletion layer reaches the defect layer, a leak current is generated therefrom, thereby extremely deteriorating the element characteristics. In the element in which leak current is not a serious problem, it does not matter if ions are implanted by using BF.sub.2, but in the case of handling a feeble electric charge, such as a solid-state image pickup device, only a slight leak current may become a dark current in the image, and a white blemish may appear unexpectedly. Therefore, if attempted to pick up an image in low quantity of light, it becomes a white blemish, and practicable solid-state image pickup device is not realized. The problem of defect is same as in the characteristic deterioration of the device by junction leak in ordinary bipolar device or MOS device, but it rarely appear as slight unevenness of the image as in the solid-state image pickup device, so that the effect of the crystal defect does not appear on the surface. In other words, the effect of injection defect is greater in the solid-state image pickup device than in other devices.
As described herein, to suppress the leak current occurring in the photo diode, it is necessary to satisfy the two requirements at the same time, that is, to reduce the interface level density and to lower the level of injection defect.